GoKawiilThe TD4 is a famous 4-bit CPU featured in the book How to Build a CPU by Kaoru Tonami. The book focuses on constructing a functional processor entirely from basic 74-series TTL logic ICs. While the book is unfortunately only available in Japanese, a friend from Japan sent me a copy along with a TD4 PCB. I believe the PCB is based on the open-source design files available on BG5DIW's GitHub repository.
"How to Build CPU" book and the PCB.
Recently, I finally found the time to build and experiment with it. The project took a few months, as I had to translate the book myself to grasp the core concepts. The overall design is simple and elegant, offering a set of 12 instructions and a 16-byte ROM (implemented via DIP switches) for programming. The board operates on 5V and can be powered via USB. Most components were sourced from local shops, though I had to order a few 74HC-series ICs online. Later, I tested the circuit by replacing some 74HC components with 74LS series chips, and it worked without any issues.
The PCB is straightforward to solder. The only SMD components are four LEDs and a Micro-USB socket. If you haven't done SMD soldering before, the USB socket might look intimidating, but you only need to solder pins 1 and 5 (the remaining pins are not used in this design).
Assembled TD4 CPU.
The most time-consuming part of the assembly was soldering the 1N4148 diodes. The book suggests using a 1S1588 or a D8-1A diode network, but these are difficult to source locally or online. Therefore, I opted for individual 1N4148 diodes. In the TD4 architecture, these diodes function as a Diode Matrix ROM. Since the "program" is stored via physical DIP switches, the diodes act as one-way gates. They prevent current from back-feeding into other parts of the circuit when multiple switches are flipped, effectively creating the logic gates necessary to decode the 16-byte memory addresses into specific 4-bit instructions for the CPU to execute.
Once assembly is complete, the TD4 is ready for immediate use; it requires no firmware or calibration. The PCB offers two power options: the easiest is via the Micro-USB port, while the second is a 2.54mm pin header. I used the pin header to attach the power leads for my logic probe, which was essential while I studied the circuit's operation alongside the book.
Functional TD4 CPU.
The TD4 is a hard-wired logic CPU, meaning it doesn't use a microprogram; every instruction is executed by direct electrical paths through the logic gates. The chipset typically consists of:
74HC161: Used for the A and B registers and the Output port.
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